Wireless Communication System Between a Vehicle and a Portable Device

1. A wireless communication system comprising: a plurality of transmitting antennas being disposed at different positions of a vehicle, each of the transmitting antennas transmitting a primary signal to a communication area according to a driving voltage thereof; a portable device receiving the primary signal when the portable device is inside of any of the communication areas; a transmission control section being integrated to the vehicle and controlling the driving voltage of each of the transmitting antennas; and an approach determination section being integrated to the vehicle and determining whether the portable device is inside of a nearby area of one of the transmitting antennas transmitting the primary signal, wherein when the approach determination section determines that the portable device is inside of the nearby area, the transmission control section changes the driving voltage of another one of the transmitting antennas disposed outside of the nearby area of the one of the transmitting antennas such that the communication area of the another one of the transmitting antennas is increased to encompass the nearby area of the one of the transmitting antennas and the primary signal is retransmitted from the another one of the transmitting antennas.

2. The wireless communication system according to claim 1 , wherein the portable device includes a signal intensity measurement section that measures an intensity of the primary signal that the portable device receives, and when the portable device receives the primary signal, the portable device transmits a second signal to the vehicle, the second signal indicating the intensity of the primary signal measured by the signal intensity measurement section, the wireless communication system further comprising a receiving section being integrated to the vehicle and receiving the secondary signal, wherein the approach determination section determines whether the portable device is inside of the nearby area based on the intensity of the primary signal indicated by the secondary signal.

3. The wireless communication system according to claim 1 , wherein the portable device includes a signal intensity measurement section that measures an intensity of the primary signal that the portable device receives, and a portable device determination section that determines whether the portable device is inside of the nearby area based on the intensity of the primary signal measured by the signal intensity measurement section, when the signal intensity measurement section determines that the portable device is inside of the nearby area, the portable device transmits a secondary signal to the vehicle, the secondary signal including an approach information indicating that the portable device is inside of the nearby area, the wireless communication system further comprising a receiving section being integrated to the vehicle and receiving the secondary signal, wherein the approach determination section determines that the portable device is inside of the nearby area when the receiving section receives the secondary signal including the approach information.

4. The wireless communication system according to claim 2 , wherein the portable device is one of a plurality of portable devices, and each of the plurality of portable devices is provided with an identification information for identifying each portable device, the wireless communication system further comprising: a storage section being integrated to the vehicle and storing the identification information of each of the portable devices; and an identification information designation section being integrated to the vehicle and designating any of the identification informations stored in the storage section, wherein the transmission control section transmits the primary signal including the identification information designated by the identification information designation section, the portable device transmits the secondary signal only when the identification information included in the primary signal matches with the identification information provided to the portable device, and when the transmission control section retransmits the primary signal from the another one of the transmitting antennas, the identification information designation section designates the identification information of the portable device located inside of the nearby area.

5. The wireless communication system according to claim 1 , wherein the nearby area is a predetermined area defined around the one of the transmitting antennas and being smaller than the communication area of the one of the transmitting antennas.